For welding of automobile bodies and automobile components, resistance spot welding, particularly direct spot welding, has been primarily used. Recently, series spot welding and indirect spot welding have also been increasingly used.
Features of the three types of spot welding described above will be explained with reference to FIG. 1.
The three types of spot welding described above are the same in that at least two overlapping metal sheets are joined together by welding.
FIG. 1(a) illustrates a direct spot welding method. As illustrated, this welding involves passing a current through two overlapping metal sheets 1 and 2 sandwiched between a pair of electrodes 3 and 4 while applying pressure to the electrodes 3 and 4 from above and below. Thus, through the use of resistance heat between the metal sheets 1 and 2, a point-like welded portion (nugget) 5 is obtained. The electrodes 3 and 4 are provided with respective force controlling systems 6 and 7 and a current controller 8, whereby a pressing force and a current can be controlled.
FIG. 1(b) illustrates a series spot welding method. As illustrated, this welding involves passing a current through two overlapping metal sheets 11 and 12 while applying pressure to a pair of electrodes 13 and 14 from the same side (in the same direction) at separate locations. Thus, point-like welded portions (nuggets) 15-1 and 15-2 are obtained.
FIG. 1(c) illustrates an indirect spot welding method. As illustrated, this welding involves pressing an electrode 23 against one (metal sheet 21) of two overlapping metal sheets 21 and 22 while applying pressure to the electrode 23, attaching a feeding point 24 to the other metal sheet 22 at a location separate from the electrode 23, and passing a current between the electrode 23 and the feeding point 24. Thus, a point-like welded portion (nugget) 25 is formed between the metal sheets 21 and 22.
Of the three welding methods described above, the direct spot welding method is used when there is enough space which allows the metal sheets to be sandwiched from above and below. In actual welding, however, it is often difficult to create enough space or to sandwich the metal sheets from above and below in a closed cross-sectional structure. In such cases, the series spot welding method or the indirect spot welding method is used.
However, when the series spot welding method or the indirect spot welding method is used for applications, such as those described above, the electrode applies pressure to the overlapping metal sheets from only one side while the other side of the metal sheets is in an unsupported and hollow state. Therefore, unlike the direct spot welding method where the metal sheets are sandwiched between electrodes on both sides, a large pressing force cannot be locally applied to a welded portion. Moreover, since the electrode sinks into the metal sheet during passage of current, the state of contact between the electrode and the metal sheet and the state of contact between the metal sheets change. This results in instability in a current path between the overlapping metal sheets, and makes it difficult to form a fused joint.
As a solution to the problems described above, the present inventors previously disclosed an indirect spot welding method involving two-stage control in Patent Literature 1. The indirect spot welding method disclosed in Patent Literature 1 is for welding a member composed of at least two overlapping metal sheets by pressing a spot welding electrode against the metal sheets while applying pressure to the spot welding electrode from one side of the member, attaching a feeding point to the metal sheet on; the other side of the member at a location separate from the spot welding electrode, and passing a current between the spot welding electrode and the feeding point. For an electrode force and a current in the indirect spot welding method, the time from turning on electricity is divided into two time periods t1 and t2. Then, a pressing force F1 and a current C1 are applied in the first time period t1, and a pressing force F2 smaller than F1 and a current C2 larger than C1 are applied in the next time period t2.
Also, the present inventors found out that the indirect spot welding method disclosed in Patent Literature 1 could be more effectively carried out by defining the pressing force F1 and the current C1 applied in the first time period t1 of the two time periods t1 and t2 after turning on electricity and the pressing force F2 and the current C2 applied in the next time period t2 as in the following expressions (2.1) to (2.4), where T (mm) represents the total thickness of the overlapping metal sheets. This finding was disclosed in Patent Literature 2.1.2F2≤F1≤5F2  (2.1)0.25C2≤C1≤0.85C2  (2.2)35T2.3≤F2≤170T1.9  (2.3)2T0.5≤C2≤5.5T0.9  (2.4)
As an improved version of the technique described above, the present inventors further developed and disclosed an indirect spot welding method involving three-stage control, instead of two-stage control, in Patent Literature 3. The indirect spot welding method disclosed in Patent Literature 3 is for welding a member composed of at least two overlapping metal sheets by pressing a spot welding electrode against the metal sheets while applying pressure to the spot welding electrode from one side of the member, attaching a feeding point to the metal sheet on the other side of the member at a location separate from the spot welding electrode, and passing a current between the spot welding electrode and the feeding point. For an electrode force and a current in the indirect spot welding method, the time from turning on electricity is divided into three time periods t1, t2, and t3. Then, a pressing force F1 and a current C1 are applied in the first time period t1, a pressing force F2 smaller than F1 and a current C2 larger than C1 are applied in the next time period t2, and a pressing force F3 smaller than or equal to F2 and a current C3 larger than C2 are applied in the next time period t3.
The present inventors further found out that the indirect spot welding method disclosed in Patent Literature 4 could be more effectively carried out by defining the pressing force F1 and the current C1 applied in the first time period t1 of the three time periods t1, t2, and t3 after turning on electricity, the pressing force F2 and the current C2 applied in the next time period t2, and the pressing force F3 and the current C3 applied in the next time period t3 as in the following expressions (4.1) to (4.6), where T (mm) represents the total thickness of the overlapping metal sheets. This finding was disclosed in Patent Literature 4.1.2F2≤F1≤3F2  (4.1)0.25C2≤C1≤0.9C2  (4.2)F3≤F2≤3F3  (4.3)0.5C3≤C2≤0.9C3  (4.4)30T2.1≤F3≤170T1.9  (4.5)2T0.5≤C3≤5.5T0.9  (4.6)
Control of current during welding in the indirect spot welding methods disclosed in Patent Literatures 1 and 3 is made possible, for example, by using a resistance spot welding controller capable of controlling a current for each cycle described in Patent Literature 5. The ranges of currents disclosed in Patent Literatures 2 and 4 can be satisfied by a normal welding apparatus.
Control of pressing force during welding in the indirect spot welding methods disclosed in Patent Literatures 1 and 3 may be achieved, for example, by using a spot welding machine disclosed in Patent Literature 6. The spot welding machine has a means for controlling a pressing force at any time on a real-time basis during passage of welding current. Commercially available resistance spot welding controllers of a servomotor pressure type include those having the function described above.
The resistance spot welding controllers of a normal type (Patent Literatures 5 and 6, commercially available) described above are designed on the assumption that an electrode force is set to a relatively large value. This means that it is not necessarily possible to stably achieve a set pressing force in a range of relatively small pressing forces preferable in the indirect spot welding method. Additionally, in the indirect spot welding methods disclosed in Patent Literatures 2 and 4, it is necessary to control a pressing force during welding with high precision.
Patent Literature 7 discloses a stationary welding apparatus which is a stationary indirect welding apparatus configured to perform indirect welding of a work. In the stationary welding apparatus, a back bar for receiving the work is disposed on a base. A drive unit that moves a movable electrode chip downward toward the back bar to apply pressure to the work is mounted on a support frame secured to the base. The drive unit is formed by an electric drive unit that uses a servomotor as a drive source. The drive unit has a casing containing a motion conversion mechanism. The servomotor is attached onto the casing. A rod that is moved up and down through the motion conversion mechanism by rotation of the servomotor protrudes downward from the casing. The movable electrode chip is attached to a lower end of the rod, with a chip holder interposed therebetween. In the casing, the drive unit is supported to be movable up and down with respect to the support frame and to be downwardly biased by a biasing means formed by a coil spring or the like.
With the electric drive unit, the indirect welding apparatus can provide fine control of pressing force. Also, when weld penetration in the work occurs by passage of current, even though the operation of the movable electrode chip may be delayed by the electric drive unit itself, a biasing force of the biasing means allows the movable electrode chip to responsively follow the weld penetration in the work. As a result, scattering of spatters can be prevented.
However, the indirect welding apparatus disclosed in Patent Literature 7 is designed based on the assumption that the back bar for receiving a work is disposed on the base. Nothing is described about indirect welding in which the opposite side of the movable electrode is in an unsupported and hollow state.
Patent Literature 7 states that by providing both the drive unit driven by the servomotor and the biasing means formed by the coil spring, fine control of pressing force can be achieved and the biasing force of the biasing means allows the movable electrode chip to responsively move when weld penetration in the work occurs. However, this indirect welding apparatus may be designed based on the assumption that a pressing force is kept constant during welding. In the case of changing a pressing force during welding as disclosed in Patent Literatures 1 to 4, when the servomotor is driven to control the pressing force, the servomotor needs to be continuously driven until the pressing force is balanced with the reactive force of the coil spring. As a result, sufficient responsiveness cannot be achieved. Moreover, since inertia that occurs when the servomotor is driven causes repetitive motion of the coil spring, a stable pressing force cannot be obtained until the repetitive motion is attenuated. No solutions to these problems are described in Patent Literature 7.